When making a change to a bug that does not require sending email notifications to any recipients, a debug message is printed at the top of the page.
No recipient addresses found in header
This is probably a sendmail error being returned from the Bugzilla::Mailer module. We should check for no recipients and then not call the appropriate mailer command or at a minimum capture the mail command output so that it doesn't display in the web UI.

I think what is happening is that a flag change is being made and when the code filters out a flag that a user/role for the bug cannot see, and there are no other changes, the email is sent without a To: recipient.
Or a person is being requested for a flag on a bug they cannot see.
Dave